Semiconductor components are made by various process steps on a semiconductor substrate. Electrical circuits are defined during the earlier portion of the process sequence. Below are the key steps in a typical metal-oxide-semiconductor (MOS) process flow:
1. Form regions of field isolation; PA1 2. Form a conductive gate over a dielectric; PA1 3. Heavily dope the source and drain regions; PA1 4. Deposit one or more dielectric layers to form an interlevel dielectric; PA1 5. Form contact holes through the interlevel dielectric where metal is able to electrically contact the source, drain, or gate regions; PA1 6. Deposit one or more metal layers and pattern the metal layers (metalization); and PA1 7. Passivate the substrate using one or more dielectric layers.
Metalization of semiconductor components is not simply a matter of making contact openings and depositing metal. Traditionally, spike formation and metal diffusion into the substrate have complicated metalization. Spiking is a phenomenon that is related to solid solubilities. When two dissimilar materials come in contact with each other, there will be an equilibrium concentration of one of the materials into the other material. As an example, pure silicon and pure aluminum come in contact. Initially, there is a smooth interface between the two. During high temperature processing such as an alloy cycle, the aluminum can support a larger amount of silicon. The silicon can go into the metal and leave a void. Conversely, the metal may diffuse through a heavily doped junction and cause a short circuit to the substrate. The result of both processes (silicon going to the aluminum and aluminum going into the silicon) is an aluminum-silicon interface that is not smooth. The process is referred to as spiking. Metal can diffuse into the substrate if the metal is capable of migrating through one or more barrier layers. Ideally, barrier layers keep the previously mentioned spiking and metal diffusion from occurring. In reality, barrier layers may not prevent spiking or metal diffusion.
Titanium nitride (TiN) is used as a barrier layer. However, TiN as sputtered suffers from two defects. First, TiN has a columnar structure. If a transmission electron micrograph were taken of the TiN, the TiN would appear as groups of columns or grains. The gaps between the columns are referred to as grain boundaries. Grain boundaries cause problems with barrier layers. The grain boundaries form a path through which the metal can migrate to reach the underlying substrate. If the metal migrates through the barrier layer, spike formation or metal diffusion into the substrate can occur.
Second, sputtering TiN itself causes a problem. A simple overview of sputtering techniques will indicate how the problem develops. A sputtering chamber is comprised of the following parts: a substrate, a target, the sputtering chamber itself, gases, and a power generator. The power generated can be direct current (DC), radio frequency (RF), etc. The generator ionizes the gas to form a plasma. The plasma is directed toward the target. In this case, reactive sputtering is utilized. The nitrogen in the plasma reacts with the surface of a titanium target to form a thin layer of TiN. In addition, the plasma hits the target causing the TiN to be stripped away from the target. The TiN coats the substrate and the walls of the sputtering chamber.
Sputtering has problems. If the sputtering occurs faster than the plasma reaction at the target (converting the surface titanium to TiN), some titanium will be sputtered form the target before it si converted to TiN. The titanium is incorporated into the sputtered film. Also, the bombardment of the plasma onto the target can generate chemical reactions. The plasma can strip the nitrogen and titanium atoms from one another with a TiN molecule. Anytime sputtering is used to deposit the TiN barrier layer, both TiN and titanium will be incorporated into the sputtered film. Within the sputtered TiN film, there will be titanium-rich areas. Compared to TiN, titanium is more reactive with the substrate and etchants. The titanium is more likely to form unwanted compounds (such as titanium silicide, TiAl.sub.3, TiAlSi, etc.) or to be etched away more readily than the TiN.
The prior art uses one of two process flows at the metalization steps. The metal can be sputtered on top of the TiN during the same evacuation cycle, or the TiN can be exposed to air before the substrate is sputtered with metal. When the TiN and metal are sputtered during the same evacuation cycle, the substrate is constantly under vacuum. The metal sputtered onto the substrate will prevent air from coming in contact with the TiN. The integrity of the barrier layer is unacceptable using this method. The metal easily passes along the grain boundaries causing spike formation or metal diffusion into the substrate.
The other metalization process flow allows the vacuum on the chamber to be broken between the deposition of the TiN and the metal. Air at about room temperature and pressure is allowed to come in contact with the TiN before the metal is deposited. Oxygen from the air reacts with the titanium in the sputtered TiN film. Titanium dioxide (TiO.sub.2) may be formed. Once reacted, titanium will no be able to form an undesired titanium compound. The oxygen is adsorbed onto the TiN grains at the grain boundaries within the TiN film. The TiN itself will not react with air at room temperature and pressure. The oxygen fills the grain boundaries thus eliminating migration paths. The oxygen adsorption is called "oxygen stuffing". The density of the sputtered film limits how much oxygen stuffing can occur. For most sputtering applications, the density of the sputtered film does not allow enough oxygen stuffing to prevent migration. Oxygen stuffing for an exposure to room-temperature air is very limited compared to a high temperature process or a plasma enhanced reaction. Most of the unreacted titanium and open grain boundaries remain within the film. Metal can pass through the sputtered TiN film.
Another prior art solution to the problem is to sputter the substrate with nitrogen-rich TiN. when reactive sputtering is used, the sputtering conditions can be changed. The change can cause a nitrogen-rich film to be deposited Nitrogen is incorporated into the TiN film during the sputter. The nitrogen may react with the titanium during subsequent high temperature processing (such as an alloy cycle). Sputtering nitrogen-rich TiN is marginally effective. The nitrogen from the nitrogen -rich areas will not react with the titanium in the titanium-rich areas if the two areas do not come in contact with one another. The TiN separating the areas may prevent the reaction from occurring. The end result is that very little of the titanium is converted to TiN, yet most of the titanium remains unreacted within the TiN film.
Still another prior art process involves the use of rapid thermal annealers (RTAs). As previously mentioned, conversion of the titanium incorporated during the TiN sputter improves the barrier layer integrity. After the substrate has been sputtered with TiN, the substrate is processed in a RTA at about 800.degree. C. to 900.degree. C. in nitrogen for 30 to 60 seconds. The high temperature causes the reaction of titanium and nitrogen to form significantly more TiN than a room temperature exposure to air. The barrier layer becomes sealed preventing spike formation or metal diffusion into the substrate. Subsequent metals may be deposited without having to worry about the integrity of the barrier layer.
RTAs suffer from a key problem; they are not production worthy. Traditionally, RTAs have been plagued by numerous equipment failures. Also, RTAs have a greater potential for warping substrates than conventional semiconductor furnaces (such as a diffusion or alloy furnace). Warping can occur if the temperature gradient across the face of the substrate is too high or if the temperature of the substrate is changed too quickly. RTAs have the potential for causing both. Many RTAs use high power lamps to heat the substrate. If the relative position of the substrate to the lamps is not proper, one portion of the substrate an be at a different temperature than another. If the temperature gradient across the substrate is too large, the substrate will warp. High temperature ramp rates may cause warping, too. A RTA can ramp from room temperature to 900.degree. C. in less than a half minute. A conventional diffusion tube takes 30 minutes or more to cover the same temperature range. Warping is much more likely to occur in a RTA than in a conventional furnace. Lithographic techniques used in semiconductor manufacturing assume that the substrate is flat. If the substrate is warped, lithographic patterns cannot be aligned over the entire surface of the substrate. Because of these limitations, RTAs are primarily used in limited production modes, such as research and development.
As will be seen, the present invention provides methods where the integrity of the barrier layer is improved by processing a sputtered TiN film in an atmospheric furnace or in a plasma reactor. The atmospheric process substantially seals the surface of the barrier layer prior to a subsequent metal deposition. The sealed surface prevents metal from getting into the grain boundaries thus preventing metal migration that results in spike formation or metal diffusion into the substrate .Anytime TiN is sputtered, titanium will be produced and incorporated into the film. Nitrogen-rich TiN by itself does not eliminate the need for subsequent processing to convert titanium to TiN. The atmospheric process uses a conventional atmospheric furnace, such as a diffusion or alloy furnace. The furnaces do not warp substrates as readily as TRAs. The furnaces are not prone to equipment failures like the RTAs.
The plasma process has a higher reaction rate of the plasma gas with titanium than exposure to room temperature air. The plasma reaction can use any gas that meets the following two constrains: First, the gas must react with titanium. Second, the gas must be able to become adsorbed onto the TiN grains at the grain boundaries within the TiN film. The gas reaction with titanium prevents the titanium from forming an undesired compound. Once the gas is adsorbed at the TiN grain boundaries, the grain boundaries "fill". Therefore, a subsequently deposited metal cannot migrate though the TiN barrier layer. Oxygen has been experimentally used and found to produce a substantially improved TiN layer. Oxygen reacts to form TiO.sub.2,and oxygen is adsorbed by the TiN grains at the grain boundaries within the TiN film. Other gases, such as nitrogen, are expected to work if they meet the previously stated constraints. Nitrogen-rich TiN does not eliminate the need for subsequent processing to convert titanium to TiN. The plasma method can use a conventional sputtering system. The system does not warp substrates as readily as RTAs. The systems are not prone to equipment failures like the RTAs. In addition, the plasma method can be utilized so that the three step (TiN deposition, reaction, and subsequent metal deposition) take place during the same evacuation cycle on the same sputter. The integrated plasma reaction method increases manufacturing efficiencies by reducing time between steps, by not requiring additional equipment, and by reducing operator handling. Less handling of the substrate results in higher yields.